Add hooks to style feeds using xslt

[Denis-de-Bernardy]

There were a few discussions several years ago about this (#1380), which essentially went nowhere.

Any odds that, with Google Reader and Feedburner on their death bed, we could have e.g. <?php do_action('rss2_stylesheet'); ?> after the xml declaration in wp-includes/feed-rss2.php, and similar hooks for other feed types, so as to offer an explanation on how to subscribe to visitors who click a feed link?

[wonderboymusic]

Duplicate of #19017.